Radio detection and ranging (radar) is used to detect, range (that is, determine a distance), and map a wide variety of objects. For example, a weather radar system on an airplane can detect and range rain or other weather events, and a military fire control radar system can detect and track a military target, such as a ship or an aircraft.
Radar can also be used to range and map objects for manufacturing purposes. For example, laser radar is particularly well-suited for manufacturing large-scale objects (such as aircraft) because laser radar allows precise measurement over a large volume. In one approach referred to as a chirped or Frequency Modulated-Continuous Wave (FMCW) radar, the radar frequency is modulated linearly (or nearly linearly).
In an FMCW radar, there is a time delay between the time when the signal is sent to an object and the time when the radiation scattered from the object is collected. As a result, there is a frequency difference between the radiation that is transmitted and the radiation that is collected. If these two signals with different frequencies are mixed, a beat frequency is observed. If the chirp is linear, then the beat frequency is proportional to the time delay and, therefore, to the range to the scattering object. If the chirp is not linear, then measuring range with a chirped radar entails measuring a sinusoidal signal that is varying in frequency (or, equivalency, in phase) in an environment of random noise.
In practical laser radar systems, the frequency or phase of the sinusoidal signal will vary—often by as much as ten percent. Because the frequency or phase of the sinusoid varies in time, a broad peak is obtained via a Fourier transform, such as a Fast Fourier Transform (FFT) of the entire interval. The broad peak reduces accuracy of measurement of frequency or phase of the sinusoid.
It would be desirable to measure accurately parameters of a signal with a narrow frequency that wanders. However, there is an unmet need in the art for a system and method for accommodating wandering in frequency or phase to permit accurately measuring frequency-and/or-phase-related parameters of a laser radar signal.